Considerable effort has been directed towards producing a drawn and ironed container from a base material consisting of black plate. Further research has also been directed towards finding an acceptable substitute for the conventional water soluble emulsified oil that is presently being used for making drawn and ironed containers.
WIth respect to the research work done with black plate, U.S. Pat. No. 3,765,206 discloses a method of drawing and ironing a container utilizing a sheet of black plate steel having metal coatings, such as tin, with different lubricity on the opposed surfaces of the sheet. The patentee contemplates that the shallow cup can be transformed into a finished container utilizing a single ironing die cooperating with a punch. Such a process is not feasible from a commercial standpoint since it would require major deviation from present day commercial machinery that is utilized for producing drawn and ironed containers. Also, it is believed that the production rate utilizing such a process would be substantially less than the present production rate for producing drawn and ironed containers.
Another proposal for producing drawn and ironed containers from black plate is disclosed in U.S. Pat. No. 3,577,753. In this process, a thin metal, such as black plate, is cleaned to remove the oxide film and is then immersed in a lubricant film composed of a phosphate coating and an organic lubricant to produce a dry film lubricant on the surface of the metal. In order to produce containers from the metal with the dry film lubricant, the patentee proposes apparatus for producing drawn and ironed containers which maintains the temperature of the punch below 50 degrees F. to prevent thermal breakdown of the lubricant. Again, such a proposal is totally impractical from a commercial standpoint since it would require a production reduction down to a few cans a minute to be able to maintain the temperature of the punch at the desired level.
Another approach for producing drawn and ironed containers from black plate is suggested in U.S. Pat. No. 4,032,678 which contemplates supplying an organic thermosetting coating on the surface of the metal and partially curing the coating prior to drawing and ironing a container.
Another area that has received a substantial degree of attention in attempting to produce black plate drawn and ironed containers is to apply a phosphate coating to the surface of the black plate. In this connection, in the past decade, several chemical suppliers have attempted to incorporate phosphates in an organic solution into which the black plate was dipped and subsequently cured. Attempts were made to draw and iron this type of material but were unsuccessful in a multistage ironing process because most of the phosphate was removed by the first ironing ring in the body maker. Therefore, there is heavy metal pick up by the second and subsequent ironing rings because the metal surface is exposed.
Other manufacturers of chemicals have attempted to use phosphate in various forms as a lubricant for the drawing and ironing process. For example, some attempts have been made to utilize an emulsion oil mixed with a highly acidic phosphate and applying this mixture to the surface of a black plate. In addition, during the ironing process, a similar mixture was recirculated through the body maker to act as a further lubricant. During tests of this particular system, it was found that tungsten carbide ironing rings could not be utilized because of the metal pick-up from the stock material and, therefore, attempts were made to use specially designed silicon carbide ironing rings. While such specialized tooling did allow for the manufacture of some containers, the end product was far inferior to what is desired and acceptable in the industry at this time. Furthermore, the highly acidic solution resulted in corrosion of the tooling and also the machine components themselves.
Another phosphate lubricant which has been proposed is disclosed in U.S. Pat. No. 3,556,867. This process consists of applying a phosphate material to the metal surface and effecting a controlled reaction to form a phosphate coating which is at least partially unreacted with the metal surface. A fatty acid soap is then applied to the phosphate surface to effect a controlled reaction between the soap composition and the phosphate coating to form a soap coating which at least in part is reacted with the phosphate coating and the metal surface.
However, after extensive experimentation and costly research, all of these attempts have been dropped and at present the only commercial use of black plate for making drawn and ironed cans is to apply a thin layer of tin to each surface which acts as a lubricant during the drawing and ironing process.
In the drawing and ironing of black plate, one of the most difficult problems is to maintain sufficient lubricant between the die and the container during the ironing process. This is the area where actual deformation takes place, and when utilizing black plate, the plastic deformation of the black plate results in developing a large amount of heat energy. This large amount of heat energy has a tendency to cause a breakdown of most water soluble emulsified lubricants which results in galling of the formed metal surface or producing metal pickup on the die rings.
Furthermore, as the process continues at production rates, the retained heat in the dies increases and may reach a temperature of more than 300 degrees F. However, the instantaneous surface temperature of the metal may even be higher. At such a temperature, most emulsified lubricants tend to lose their lubricating capability. Also, when the emulsified oil or other lubricant is mixed with the water coolant, the cooling capability of the water is decreased.
This high temperature can also produce stripping problems. Stripping problems relate primarily to shrinkage of the container on the punch after the last ironing step and before stripping actually takes place which results in large frictional forces between the punch and the container. Stripping problems are most acute where the temperature gradient between the punch and the container is high, which is produced by the large frictional forces that are developed as the black plate wall is being reduced in thickness.
In this respect, numerous attempts have also been made to solve such problems. One proposal is disclosed in U.S. Pat. No. 3,670,543 which suggests roughening the surface of the black plate metal or low carbon steel to produce minute depressions in the surface so that the lubricant can be applied to the roughened surface and retained in these small depressions during the drawing and ironing operation.
Thus, while the prior art is replete with suggestions for solving the two basic problems encountered in attempting to draw and iron black plate metal, no acceptable commercial process has been developed which is economically feasible.